Known fluoroelastomer compositions comprise the fluorocarbon elastomer or gum, a metal oxide, a filler, curing agents and processing aids. Converting the rubbery raw gum fluoroelastomer to vulcanizates requires primary crosslinking or curing agents such as the amines, dithiols, peroixdes or certain aromatic polyhydroxy compounds. Alternatively, radiation can be used. With each of these systems a basic metal oxide is required, such as the oxides of magnesium, calcium, lead or zinc, as an acid acceptor. Fillers are employed for their usual purposes those being to reinforce the elastomer and reduce cost. Processing aids are also employed for conventional purposes.
To compound a fluoroelastomer for coating purposes, it is customary to mix the gum with a desired filler and a metallic oxide on a mill, working the additives well into the gum. After removal from the mill, a solvent such as a low molecular weight ester or ketone is added followed by an aliphatic amine. The amine initiates curing which requires that the composition be utilized within several hours.
In another curing system, a ketimine is added to the foregoing mill mix with the solvent. The resulting mixture is relatively stable so long as moisture is not present. Ketimines are also utilized to cure epoxy resins and in the presence of moisture from the air or otherwise, breeak down to form an amine and a ketone. Once the amine has been released, the fluoroelastomer begins to cure, providing a working life of several hours.
Yet another system includes the addition of the curative with the gum, filler and metallic oxide on the mill. Curatives such as hexamethylene diamine carbamate, ethylene diamine carbamate or dicinnamylidene-1,6-hexane-diamine, commonly referred to as the DIAK's, are used. Care must be exercised that the temperature on the mill does not rise too high in order to avoid premature curing. The resulting mixture can then be processed on conventional apparatus or it can be mixed with a solvent to be used for coating work. Heat completes the final cure in this system as it also does with the preceding systems.
When used as a coating, several problems exist. A primary one is adhesion; pretreatment of the substrate is required including cleaning and priming operations. Another problem is that settling of the metallic oxide will occur during use of the coating material, giving a nonuniform dispersion of the metallic oxide and nonuniform cure. Working life is usually relatively low requiring the manufacturer to compound the fluoroelastomer and use it the same day, often within hours. Use of ketimines, for instance, necessitates airless spraying and closed dipping tank systems in order to avoid premature curing prior to the application. Where solvents are not employed, for production of solid products, mill mixing rarely results in homogeneous mixtures of the metallic oxide and curative such as DIAK, throughout the gum.
The foregoing fluoroelastomer systems typify the state of the art and although the compositions have been utilized to form solid products as well as coated products, use as a coating has had its shortcomings. Coatings obviously provide a fluoroelastomer surface without the expense of the entire article being a fluoroelastomer. In other instances, where size, strength or location of the article militates against solid elastomer construction, a coating is the only manner in which the fluoroelastomer can be employed.
Of the systems and techniques known to me, none has provided a composition readily adherable to a plurality of substrates, providing an abrasion resistant coating and without loss of the inherent chemical resistance possessed by the fluoroelastomer. Ideally, a thinner film, on the order of one or more mils (0.025 mm) thickness, that could adhere to a variety of substrates or envelop them, would enable fluoroelastomers to be used in applications where heretofore they have been unfit due either to high costs or poor adhesion.